Conventionally, for the purpose of enhancing comfort of passengers in vehicles such as automobiles, there have been made various attempts to restrain the vibrations or noises from penetrating into the vehicle compartment by disposing various vibration insulators at those parts probably serving as sources of vibrations or noises.
For example, in relation to an engine which is a main source of vibrations and noises, vibration-insulating rubbers have been used for such component members as the torsional damper and the engine mount, to thereby absorb the vibrations during driving of the engine and to restrain both penetration of vibrations or noises into the compartment and diffusion of the noises to the peripheral environments.
As fundamental properties, such a vibration-insulating rubber is required to have a strength characteristic for supporting a heavyweight body such as the engine and a vibration-insulating performance for absorbing and suppressing vibrations. Further, a vibration-insulating rubber for use in a high-temperature environment such as an engine room is required to have not only a low dynamic-to-static modulus ratio and an excellent vibration-insulating performance but also high thermal resistance, ozone resistance, and permanent compression set. In particular, the temperature of an engine room tends to be increased in recent years along with high output of engine and reduction in engine room space due to increase in vehicle interior space. Consequently, the vibration-insulating rubbers for use in automobiles have come to be desired to meet severer requirements in regard of thermal resistance and the like.
Further, automobiles are also used in high latitudes, and thus the vibration-insulating rubbers for use in automobiles are required to have low temperature characteristics, in addition to the aforementioned properties.
In order to impart the vibration-insulating rubber with such excellent properties in a comprehensive manner, active developments have been made on the rubber composition of the vibration-insulating rubber and on crosslinking systems and other additives to be contained by a predetermined amount, and a number of patent applications have been filed therefor. Of those numerous patent applications, some have actively employed a bismaleimide compound as an improved crosslinking system.
For example, Patent Literature 1 discloses a rubber composition having a rubber component compounded with sulfur, bismaleimide, and a specific carbon black, so as to be excellent in thermal resistance, low dynamic-to-static modulus ratio, and the like.
Further, Patent Literature 2 discloses the use of a bismaleimide compound and a thiazole vulcanization accelerator, which contributes to obtaining a rubber composition being excellent in thermal resistance, low dynamic-to-static modulus ratio, and durability.
However, the rubber compositions of PTL 1 and PTL 2 are still slightly inferior in terms of low dynamic-to-static modulus ratio, elongation fatigue resistance, and low temperature characteristics, even though both are reasonably excellent in thermal resistance and low dynamic-to-static modulus ratio. Further, in order to retain spring properties strongly required for vibration-insulating rubbers, it is necessary to suppress the rate of change of the modulus to minimum, and also to further improve thermal resistance.
To this end, attempts have been made to lower the dynamic-to-static modulus ratio, and to improve elongation fatigue resistance and low temperature characteristics by including sulfur as a vulcanizing agent, a specific sulfur compound, and a bismaleimide compound. For example, Patent Literature 3 discloses a vibration-insulating rubber composition containing: sulfur in an amount of not more than 0.5 parts by mass; a sulfur-containing compound of a specific structure in an amount of 0.5 to 2 parts by mass; and a bismaleimide compound in an amount of 0.5 to 3 parts by mass, each based on 100 parts by mass of diene rubber.